Download Computation in a Topological Quantum Field Theory

Computation in a Topological Quantum Field Theory
Daniel Epelbaum and Raeez Lorgat
December 2015
Abstract
This report investigates the computational power of the particle excitations of topological phases of matter, as modeled by
a Topological Quantum Field Theory. We ask two basic questions: i) how does the computational power of these excitations
change as a function of the genus of a fixed 2-dimensional space-time? and ii) independent of any particular space-time,
what structural properties of a TQFT govern its computational power? When restricted to a space-time with space-like
degrees of freedom represented by a surface of genus g, we answer the first question by observing a qg -fold degeneracy in
the ground state of the TQFT resulting from the presence of abelian anyons with exchange statistics a q-th root of unity. Such
a resource is a topologically fault-tolerant quantum memory; however, braiding of the abelian anyons dos not afford the
implementation of arbitrary unitary transforms. We address the second question by studying the algebraic model of anyons
via Tensor Categories, and discuss what is known as of December 2015.
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1.1
Computation in a Topological Quantum Field Theory
Computing with Anyons
A characteristic property of quantum particles is the indistinguishability of identical particles. All photons in the world
share the same dynamical behaviour: for any system composed of multiple identical photons a permutation of the positions of any two photons cannot have any effect on the dynamics of the system as a whole. It follows that Sn the symmetric
group on n elements acts on an ensemble of n identical particles as a symmetry of the composite system; more precisely,
the system’s wave function remains unchanged up to a total phase. In this way Unitarity of our physical theories shows
the resulting transform of the wave function is an element of the group U(1) i.e. transforms by scalar multiplication by a
complex phase eiθ .
It is well-known that particle exchanges in three-dimensional space transform in precisely two flavours, each corresponding
to one of two 1-dimensional irreducible representations of Sn . If the particles are bosons (e.g. photons), then an exchange of
two particles is represented by the identity symmetry: the wave function is invariant, and the particles obey Bose statistics.
If the particles are fermions (e.g. electrons in a metal), then the action of exchange is represented on the wave function as a
multiplication by -1 i.e. the wave function changes sign. Correspondingly, the particles obey Fermi statistics.
However, in a space-time restricted to two spatial dimensions we instead find a wealth of exotic particle statistics. The
restricted topology of a surface imposes a topological change to the symmetry in exchanging two particles; the exchange
symmetry group is no longer modeled by the symmetric group Sn , but instead by the braid group Bn (See Appendix
A), as depicted in Figure 1.1. Vertical displacement is in the time direction and the n particles are subjected to movement
exchanging their positions from ai to bi . The braids are formed from the individual particle’s world-lines.
Figure 1: Particle world-lines tracing out a braid. From [14].
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Contrary to Sn , the infinite group Bn has infinitely many one-dimensional irreducible unitary representations, each
corresponding to a choice of phase eiθ . We see that for θ = 0 and θ = π we recover bosonic and fermionic particles, but for
every other choice of θ we find a new particle. As these particles can obtain any change of phase upon permutation they
have been dubbed anyons.
Furthermore, in certain topological quantum field theories (henceforth TQFTs) we find that the higher dimensional representations of Bn manifest themselves in an even more exotic type of particle; in contrast to those anyons that transform
according to a one dimensional unitary representation i.e. according to a phase eiθ in the abelian group U(1), these nonabelian particles transform according to a representation Vd of Bn in some non-abelian unitary group U(d) where d > 1
is the dimension of the corresponding representation. Fixing Vd , a pair of anyons of type Vd with movement confined to a
surface trace out a topologically non-trivial braid formed by the particle’s world-lines. This braid then corresponds via the
representation to an element U(d). These anyonic particles carry an intrinsic computational resource that is purely topological in nature. According to the unitary representation of Bn that they are modeled by, each particle carries internal degrees
of freedom, and this state can be transformed by appropriately braiding the particles by moving them in 2-dimensional
space-time. It is this computational resource of a TQFT that this report investigates.
Remark (On our treatment of TQFTs). In order to not move too far afield, we focus on the strictly 2-dimensional part of
a 2 + 1-dimensional TQFT i.e. its anyonic excitations and corresponding braiding statistics. Furthermore, since a Unitary
Modular Tensor Category (UMTC) in the sence of Tuarev corresponds uniquely to a TQFT, we will instead work with
UMTCs. Appendices B and C contain an informal discussion of TQFT and UMTCs in general. For a complete treatment,
see [12] and [13].
The rest of this report is divided into two sections: a review of the structure and classification of 2+1-dimensional TQFTs
as it relates to the computational power of the TQFT, followed by a discussion of a fault-tolerant quantum memory in the
topological degeneracy held by abelion anyons braided on a genus g surface. Here we find that changing the topology of
the surface does not change the range of computations possible, but instead adds qg degrees of freedom not accessible via
local operations.
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2.1
The Computational Power of a TQFT
Universality and Density of the Braid Group Representations
Given that each UMTC C (see Appendix C) models a topological state of matter, it follows that the computational power of
the nonabelian statistics of an anyon native to a given 2 + 1-dimensional TQFT modeled by C is governed by the algebraic
structure of C . Much like a choice of gate set in the standard Quantum Circuit Model may or may not afford efficient approximation of any quantum algorithm, the choice of UMTC governs the availability of a non-abelian anyon with braiding
statistics supporting implementation of any quantum algorithm.
More precisely: given an object V in C , the structure of a UMTC affords a homomorphism from the group algebra of
the braid group
CBn → End(V ⊗n )
acting on braid group generators σi (See Appendix A) as
⊗i−1
⊗n−i−1
σi 7→ IdV
⊗ σV,V ⊗ IdV
where σV,V denotes the braiding homomorphism
V ⊗ V → V ⊗ V.
Furthermore, naturality of the braiding homomorphism implies compatability of braiding with the Hilbert Space structure
on End(V ⊗n ). It follows that the left action of End(V ⊗n ) on itself induces a unitary representation
Bn → U(End(V ⊗n )).
Thus for a given anyon type modeled by V, choosing m, n ∈ N anyons to encode one and two qubits respectively yields
braid group representations of Bm and Bn in End(V ⊗m ) and End(V ⊗n ). If each of these representations have their images
dense in the special unitary group, then we can apply the Solovay-Kitaev theorem to approximate any desired unitary to precision in a braid word of length poly(log( 1 )).
Example 2.1.
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Freedman, Larsen and Wang characterize the images of the Jones representations of the Braid group in [7], deriving as
corollary in [8] that the UMTC C (sl2 , eπi5 ) consisting of highest weight representations obtained as a subquotient of the representation category of the Quantum Group Uq sl2 specialized at q = eπi5 is universal. This particular UMTC corresponds
to the SU(2)-Chern-Simons-Witten TQFT at level 3, or equivalently the Fibonacci UMTC F described in Appendix B.
Example 2.2. For an example lacking universality, by Jones in [10] the SU(2)-Chern-Simons-Witten TQFT at level 2 has braid
group representations with image factoring through a finite group. In this case, the corresponding UMTC is C (sl2 , eπi4 ).
2.2
Universality of a Given UMTC
Due to the divergence in computational power of different UMTCs, it is natural to ask for necessary and sufficient conditions for universality of the computational in a TQFT arising from a UMTC. As of December 2015, this remains an open
problem. Recent progress by Etingof, Rowell and Wang have led to a conjectural characterization of UMTCs with braid
group representations having exclusively finite image [5]:
Definition 1. A UMTC C has property F if the associated representations of Bn on the centralizer algebras End(V ⊗n ) have finite
image for all objects V and all n ∈ Z.
In any UMTC C we can associate to objects X, Y ∈ Obj(C ) and a map f : X → Y a number called the trace of f denoted
tr f. In particular, for the identity map IdV : V → V we can define the dimension dim(V)
P = tr IdV . For example, for any
V ∈ Vk , dim(X) ∈ N is the standard vector space dimension. Denoting by dim(C ) = i dim(Vi )2 the global quantum
dimension of C , then the conjecture, verified for all known examples, is
Conjecture 2.1. A UMTC C has property F if and only if dim(C ) ∈ N.
Returning to our standard example:
Example 2.3. Recall that the Fibonacci UMTC F was shown to be universal in [7]. Here we have two anyons 1 and τ, with
dimensions
√
1+ 5
.
dim(1) = 1 and dim(τ) =
2
It follows that
√
5+ 5
,
dim(F ) =
2
in accord with F being universal and hence not having property F.
2.3
Classification of UMTCs of Low Rank
Recall the rank of a UMTC C is the number of simple objects in C . In [4] Wang conjectured that UMTCs could be classified
not only as representation categories of Quantum Groups, but also directly according to their rank. This conjecture was
recently proven in [1] and is now called the Rank Finiteness Theorem:
Theorem 2.2. There are finitely many isomorphism classes of UMTCs of fixed rank r.
The theorem is directly analogous to a classical result of Landau’s: for a fixed number n ∈ N, there are only finitely many
finite groups G with exactly n irreducible complex representations. The proof follows from analyzing the class equation
|G| =
n
X
[G : C(gi )],
i=1
dividing by |G| to yield the diophantine equation
1=
n
X
1
xi
i=1
which has finitely many solutions in the positive integers thus implying a bound on |G|; it follows there are only finitely
many such G with n irreducible complex representations. Similarly, for a rank n UMTC C , the proof of the rank finiteness
theorem in [1] analyses the equation
n
X
dim(C ) =
dim(Vi )2
i=1
in order to produce a bound on dim(C ) implying a finite possible set of fusion rules for C ; a further analysis then reveals
finitely many UMTCs having a fixed fusion rule, implying the result.
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The Rank Finiteness Theorem suggests the feasability of a classification of UMTCs by rank. The process of classification can be understood from the axiomatic specification of a UMTC: each axiom imposes a polynomial constraint with
Z-coefficients, equating the classification of UMTCs with counting points on certain algebraic varieties (solution sets to
polynomial equations). As of December 2015, UMTCs of rank 4 and lower have been classified via the galois groups associated to them, while in rank 5 all that is known is a list of possible fusion rules. The difficulty of the problem in rank 5 and
greater can be described in terms of the higher dimensionality of the algebraic and arithmetic varieties involved [4].
According to the classification in [4], there are 70 UMTCs of rank less than or equal to 4, 10 of which are prime. The
rest can be obtained from these 10 by applications of (categorical) direct sum and symmetry transformations. Table 1 below
describes these, noting that 9/10 of the prime UMTCs are known to arise from a Quantum Group; here we label them
according to the construction in [11]. We also explicitly name two UMTCs discussed in this report: Fibonacci and Toric
Code. Column 3, row 4 is the only UMTC not known to arise as a coset construction from the category of representations of
a Quantum Group; see [4] for details.
[1] Trivial A
[2] SU(2)1 A
[2] (G2 )1 , also called Fibonacci F NA [U]
[2] SU(3)1 A
[8] E82 NA
[2] (A1 , 5) 1 NA [U]
2
[5] Spin(16) Toric Code A
[4] SU(4)1 A
[4] (G2 )1 × SU(2)1 [U]
[2](G2 )2 NA [U]
[3](G2 )1 × (G2 )1 NA [U]
Table 1: The ith column classifies rank i UMTCs. In each box we record: [n] the number n of distinct theories in the
isomorphism class; the lie group G and level k specifying the TQFT Gk by which the UMTC arises; whether the anyons are
abelian (A) or non-abelian (NA); and [U] the presence of a universal braiding anyon.
An example of a topological phase of matter is seen fractional quantum Hall liquids. These are electron systems confined
to a disk and subjected to a perpendicular magnetic field at extremely low temperatures. Electrons in the disk, pictured
classically as orbiting concentric annuli around the origin, organize themselves into a topological order. In this way the
classification of UMTCs is akin to obtaining a periodic table of elements for the topological phases of matter.
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Toric Code and Abelian Anyons
We now restrict our attention to abelion anyonic particle dynamics on a surface of genus g. To do this we first look at a
model of qubits on a surface with genus g = 1 i.e. a torus. We then generalize this case to arbitrary genus.
3.1
Toric Code
The toric code, introduced by Kitaev, is an example of a symplectic or stabilizer code. Stabilizer codes are quantum analogues of classical linear codes. In general such a code works by selecting a set of “check operators”, analogous to checksums, and encoding qubits in states belonging to the stabilizer space for each of the check operators. This allows for
correction of a particular set of errors by first measuring a “syndrome” to determine the type of error—essentially checking
to see if the state is still in the stabilizer space. By measuring the syndrome, and not the information in the bits directly, we
can reconstruct the error and reverse it without damaging our state.
Consider an r × r lattice embedded on a torus. On each edge of the lattice we place a qubit (or a spin- 12 degree of
freedom). We then have two types of check operators. The first type we will call “star operators”. These are operators
Axs =
Y
σxj
j∈star(s)
where star(s) for some vertex s is the set of edges, or equivalently qubits, adjacent to that vertex. The second type we will
call “face operators”. These are defined by
Y
Azu =
σzj
j∈boundary(u)
where boundary(u) is the set of qubits around a given face u. For the purposes of a stabilizer code, we now want to find
what the stabilizer space of these operators looks like. We have 2r2 qubits, and 2r2 check operators. However, we have two
relations among the check operators i.e.
Y
Y
Axs = I =
Azu .
s
u
4
We conclude that the stabilizer space has dimension 22 = 4.
Furthermore, each operator only acts on 4 qubits, and each qubit is only acted on by 4 operators. In addition, because
the operators all commute (star operators and face operators commute because a boundary and a star either share 0 or 2
qubits) we can apply the syndrome measurement in a constant depth circuit. We can check that the errors undetectable by
this code are only those errors that encompass an entire cycle i.e. an element of the homology group of the torus. We can
show this either by working with the state space directly or by understanding the toric code in terms of anyonic particles.
3.2
Anyons and The Toric Code
Consider the same system as above—an r × r lattice with a qubit on each edge embedded on a torus. When we view this as
a physical system we are led to the Hamiltonian
X
X
H=
(I − Axs ) +
(I − Azu ).
s
u
The null space of this operator is exactly the stabilizer code space of the toric code. Because the operator is nonnegative, we
then see that the stabilizer space of the code is precisely the space of minimal energy for this Hamiltonian. It follows that
the excited states, having non-minimal energy, then form the orthogonal complement.
Now suppose we have the ground state |ξi. We want to consider some minimal excited state |νi, and to this end we can
consider the excited states as ordered by the number of conditions they violate. For the state |ξi we have that for all faces
and vertices Axs |ξi = Azu |ξi = |ξi. Due to the relations described above, the number of violated conditions of a given type
(either face or star) must be even. Let us consider the simplest case of two violated conditions at vertices s and p. Then we
have at these vertices Axs |νi = −|νi and the same for p. It is standard terminology to say that we have two quasiparticles at
s and p. First, we note that we can generate the state |νi from the state |ξi. We simply apply a product of σxj for all j along
a path from s to p. We can also move quasiparticles along a path by applying products of σxj along that path. Similarly, we
can talk about quasiparticles on faces by considering violated conditions at faces. Again these can be created by products of
σzj for paths along the “dual lattice” i.e. the lattice given by thinking of faces as vertices, and connecting those vertices that
correspond to adjacent faces.
We now consider what happens when these two types of particles interact. Suppose we have two vertex quasiparticles
at s and p, and two face quasiparticles at u and v. We consider the action of moving one of our face quasiparticles around
a closed loop containing the vertex p. Moving this quasparticle around a closed loop is an application of star operators at
every vertex inside the loop. All of these act as the identity except for the star operator at vertex p which flips the sign of
our state. Moving one particle around another results in a phase change, even though the particles don’t directly interact!
Finally, because we are working on a torus and not a plane, there is another action we can consider. Consider 4 operators,
Z1 , Z2 , X1 , X2 , where the Z1 , Z2 operators are products of σz around the two different nontrivial loops of the torus, and
X1 , X2 are products of σx along nontrivial loops. The commutation relations between these operators are
X1 X2 = X2 X1 Z1 Z2 = Z2 Z1
X1 Z1 = Z1 X1 X2 Z2 = Z2 X2
X2 Z1 = −Z1 X2 X1 Z2 = −Z2 X1 .
The operators X1 , Z2 and Z2 , X1 can each be thought of as acting as σx and σz operators on the two different qubits embedded by the toric code—in other words they act on the 4-dimensional space of ground states of the Hamiltonian. One way
to think about this is to consider creating a pair of vertex and a pair of face particles. If we move one of the vertex particles
around a nontrivial cycle, and then annihilate it with the other vertex particle, essentially performing X1 we get a phase
change, since we have also moved the particle in a closed path around a face particle. However, X1 has to commute with
the operators that create face particles, and so if we first perform X1 and then create a pair of face particles, we should get
an already-flipped state. In other words, the ground state “remembers” the topological behavior of past anyons.
3.3
Abelian Anyons on a Surface: A Generalization
What we have described above is simply a pair of particles such that when one is moved around the other the state vector
acquires a phase flip of −1. It follows that these particles can be viewed as anyons with exchange statistics which are a
fourth root of unity. Now consider a more general case: anyons on a torus with exchange statistics given by some eiθ where
p
. These are also known as rational anyons, since the phase change given by their exchange is a rational multiple of
θ = πq
π. We will restrict our study to rational anyons.
Rather than looking at operators of particles moving around one another, we instead look at two operators C1 and C2 ,
which are analogous to our Xi and Zi operators above. The operator C1 is given by the creation of a pair of anyons, moving
one around one nontrivial cycle of the torus, and then fusing the anyons and annihilating them. The operator C2 is the
5
analogous operator for the other non-trivial loop. As above, these will act nontrivially on the ground state of the torus, and
−1
again such action is a topological behavior. We consider the commutator [C1 , C2 ] = C−1
2 C1 C2 C1 . Topologically what we
have done is to move one particle in a loop around another, and this is equivalent to two exchanges. It follows that
[C1 , C2 ] = ei2θ .
Consider some eigenvector |ψi of C1 with eigenvalue eiα (since Ci ’s are unitary their eigenvalues have this form). Then we
have the relation
[C1 , C2 ]|ψi = ei2θ |ψi
C2 eiα |ψi = C1 C2 ei2θ |ψi.
Rearranging terms we can get
C1 (C2 |ψi) = ei(α−2θ) (C2 |ψi).
In other words C2 acts as a shift operator on eigenvalues of C1 . Furthermore we see
p
[C1 , C2 ]q = eqi2π q = 1.
In summary, moving abelion anyons on a torus yields an additional q-fold degeneracy that does not exist when quasiparticles with the same exchange statistics move on a plane.
There is another way we can generalize this picture, and that is passing to surfaces of higher genus. Suppose then that
the spatial degrees of freedom form a surface of genus g. The above analysis depends only upon the existence of 2 nontrivial loops on the torus. A genus g surface has 2g non-trivial loops. We organize these loops into pairs—one pair for each
hole in our surface (a genus g surface can be thought of as a connected sum of g tori, or a surface with g holes). To each
of these loops we associate an operator Cji where the i = 1, 2 and j ranges from 1 to g. Operators corresponding to loops
in different pairs commute, and operators corresponding to the same loops commute. The only nontrivial commutator is
[Cj1 , Cj2 ] = ei2θ . We conclude then that on a surface of genus g, we have g copies of the q-fold degeneracy from the topology of the torus. In other words our ground state space has a qg -fold degeneracy above and beyond what is present when
abelians are braided on a genus 0 surface.
As the analysis shows, these abelian anyons on a genus g surface may have a finite dimensional topological degeneracy,
but their braiding does not allow the implementation of arbitrary unitary transforms. At best we find a stable ground state
protected from local errors.
4
Conclusion
This report discussed the computational power of a topological quantum field theory. We reviewed two basic questions: (i)
How does the toplogy of space-time affect the computational power of a TQFT and (ii) independent of a particular spacetime, what intrinsic structural properties of a TQFT govern its computational power? The observation that abelian anyons
on a genus g surface generalizes the toric code answers the first question: a non-trivial 2-D space-time topology provides
an exponential (in the genus g) degeneracy in the ground state of the TQFT, a resource that in principle could serve as a
quantum storage robust to local error, but cannot be used to implement any transformation not already accessible to anyons
with comparable statistics on a genus g = 0 surface. To answer the second question, we appealed to the algebraic model
of anyonic quantum computation via the Tensor Category formalism. By relying on our intuition from classical group theory to guide our understanding of their quantum generilizations, we have seen how the correspondence between UMTCs
and 3-dimensional TQFTs, affords an algebraic and number theoretic analysis of the computational regime modeled by any
TQFT. Recent deep results in this direction such as the Rank Finiteness Theorem and classification of UMTCs of low rank
are the fruits of this approach: we are able to derive an explicit periodic table of elements for topological states of matter,
indexed by the nature of the anyonic excitations provided.
Several open questions present themselves: we still do not have a good characterization of when a UMTC/TQFT has a
universal computational model. Furthermore, restricting to those cases where the computational model is not universal,
we do not have a good characterization of what computational power they do provide: at best we have a conjectural characterization of when arbitrary braiding of anyonic excitations implements at most a finite subgroup of the unitary group.
Basic obstructions to answering these questions are a lack of understanding the relationships between the objects involved.
For example, it’s expected—but not yet known—that there are UMTCs that do not arise from Quantum Groups; in practice
all known UMTCs can be constructed via some procedure from a Quantum Group. This could be rephrased as a gap in
our understanding of the representation theory of a general Quantum Group. Given how basic computational questions directly translate into open problems concerning these relatively young fields, we expect new advances to directly contribute
towards a deeper understanding of quantum computation arising from a TQFT in the years to come.
6
Appendix A The Braid Group
The braid group Bn on n strands, is a generalization of the symmetric group Sn on n symbols. It has an intuitive presentation as braids of n strings. Formally it is presented by n − 1 generators σi which corresponds to crossing the ith strand
over the i + 1th strand. For example, the braid
is given by generators
σ1 σ2 σ−1
1 .
Such a sequence of generators is called a braid word. This set of generators has a set of relations:
σi σj = σj σi σi σi+1 σi = σi+1 σi σi+1
where |i − j| > 2. Such relations are easy to see if you draw out the corresponding diagrams.
Appendix B: The Fibonacci Theory F
We now introduce one of the simplest non-trivial TQFTs which we denote F , known as the Fibonacci theory in the literature
(See pp. 21 in [6]). The purpose of explicitly describing F here is to provide a concrete example to help motivate the abstract
language of UMTCs used in this report.
We specify F by the structure of the anyonic excitations it supports in any 2 dimensional slice of space-time, as well as
the resulting exchange statistics. In this model, there are only two different types of anyons, the vacuum (or absence of an
anyon) denoted 1 and the non-abelian anyon τ. Part of the data specifying a TQFT is what is known as a fusion rule, which
specifies what happens when two particles are brought together. Fusion can be considered as identifying the two anyons as
a composite particle and identifying the resulting statistical behaviour of the ensemble. For example, fusing two fermions
results in a boson. In the case of F :
τ × τ = 1 + τ,
τ × 1 = τ,
1 × τ = τ,
1 × 1 = 1.
These rules should be read, for example in the first row, as stating that fusing two τ particles results in a superposition of
two outcome states: the first state being the annihilation of both particles (the outcome being the vacuum) and the second
state being the formation of another τ particle. The fact that this fusion space has dimension higher than 1 and so allows
for superpositions is equivalent to τ being a non-abelian anyon, a fact that has only recently been proved; see [2].
τ τ τ τ τ τ
1 or τ
Figure 2: Fusion of multiple τ particles. From [15].
The computational power of F is now made apparent in the process of fusing n anyons of type τ. Consider a line of τ
particles as depicted in figure 2 and proceed to fuse these particles in a step-wise fashion. We begin by fusing the first two
particles, and then continue by fusing the outcome with the remaining particles incrementally. To each step i we assign an
index ei that indicates the outcome of the fusion at that step as being either 1 or τ. The states |e1 , e2 , . . . , en−3 i belong to
7
what is called the fusion Hilbert space of the τ anyons, denoted Hn . In principle, there are 2n−3 possible outcomes of ei ’s,
but not all are allowed by the fusion rules. For n = 1, we deal with the impossible case of the vacuum turning into a τ
anyon, so
dim(H1 ) = 0.
For n = 2 we see τ as input and output going through a trivial process so
dim(H2 ) = 1.
Next, the possible outcomes are 1 or τ, giving
dim(H3 ) = 1,
but at the very next step we see that there are two possible ways of yielding a τ from two different processes and so
dim(H4 ) = 2.
Continuing this way we see that
dim(H5 ) = 3,
and so on, yielding the sequence
0, 1, 1, 2, 3, 5, 8, 13, . . .
√
which grows proportionally to φn where φ = 1+2 5 . Thus the fusion state space Hn of internal degrees of freedom of n
anyons of type τ grows exponentially with n. In order for this to be a viable computational resource, we ideally want to
be able to implement arbitrary unitary transforms on Hn . As mentioned in the previous section, this is accomplished by
braiding two τ anyons around one another as in Figure 2.3.
b
a
b
a
Ra,b
b
a
= Rcb,a
=
c
c
c
Figure 3: Braiding of two τ particles. From [15].
The process of doing so is a unitary transform on those basis vectors in the fusion space spanning possible fusion
outcomes c according to the fusion rules governing a and b in the particular TQFT. Hence, this braiding matrix is part of
what specifies a TQFT. In the case of F again, we find that exchanging any two τ anyons acts as the matrix
!
4πi
e 5
0
Rτ,τ =
.
2πi
0
−e 5
Given an exponential state space, rules for braiding, and rules for fusing, we can now see that in order to encode a logical
qubit as in the standard Quantum Circuit Model, we might employ four τ anyons. There are two distinguishable ways
these anyons can be fused that can encode the qubit states: |0i = |τ, τ → 1i and |1i = |τ, τ → τi. The possible logic gates are
accomplished by forming arbitrary braids amongst these four qubits, with behaviour governed by Rτ,τ . Fusing and reading
the outcome then correspond to a measurement in the QCM.
As our goal is to study the general computational power of a given TQFT, we find it fruitful to abstract the data presented
here in the form of an algebraic model of anyons given by Tensor Categories as described in [12]. As described in Section
2, not only does the language of tensor categories simplify the description of a TQFT, but it also affords us algebraic and
number-theoretic tools to analyse the computational power carried by any given TQFT.
Appendix C: TQFTs and Unitary Modular Tensor Categories
In the example furnished in Appendix B, a model of computation arose from a TQFT after specifying:
• a set of objects corresponding to the available types of anyons
• rules for fusing these objects pairwise
8
• rules for braiding these objects pairwise
We now capture this data in an algebraic structure known as a Unitary Modular Tensor Category. We include the concise
definition for readers familiar with tensor categories, but for those not, immediately after we turn to recast the definition in
terms of two familiar objects: i) the category Vk of finite dimensional vector spaces over the field k and ii) the TQFT F .
Definition 2. A Unitary Modular Tensor Category (abbreviated UMTC) is a semi-simple ribbon category C satisfying the following
properties:
• C has only a finite number of isomorphism classes of simple objects.
• C is modular i.e. has non-degenerate S-matrix.
We refer the interested reader to [12] for a detailed exposition of the terminology used here. Modularity, though important for the dictionary between TQFTs and UMTCs, will not play an immediate technical role in any of our subsequent
discussions, so we omit covering it here. Instead, for our purposes we now quickly describe the structure of two UMTCs:
Vk and F .
C is a Semi-Simple Ribbon Category with finitely many isomorphism classes of simple objects
That C is a category means that C can be thought of as a collection of objects Obj(C ). For example, in the category Vk of
finite dimensional vector spaces over the field k, Obj(Vk ) is the set of isomorphism classes of vector spaces over k of finite
dimension; i.e. one object for each n ∈ N. In the case of F , = {1, τ} ⊂ Obj(F ), but the full collection of objects includes
additional objects that can be constructed from these two, as soon shall be seen.
That C is a Semi-Simple Ribbon Category means that C has natural duals, natural tensor products, and a natural braiding structure on Obj(C ). In the case of Vk , we see for any V, W ∈ Obj(Vk ) these constructions are the familiar dual vector
spaces:
V → V∗
tensor product
V ⊗W
and braiding map coinciding with commutativity of tensor product
V ⊗ W = W ⊗ V.
In F the natural duals on Obj(F ) are particle-to-antiparticle correlation
1 = 1̄
and
τ = τ̄
i.e. both 1 and τ are their own antiparticle (i.e. are self-dual). Tensor product then corresponds to fusion e.g.
τ × τ ∈ Obj(F )
is the composite particle with behaviour governed by the collective statistical behaviour of two τ particles, and finally a
natural braiding map given by
Rτ,τ : τ × τ → τ × τ.
Finally, that C is semi-simple means each tensor product of objects can be decomposed into a direct sum of a distinguished
class of simple objects. Here simple should be taken in the sense of an irreducible representation, prime number etc: a
simple object is one that cannot be decomposed into a direct sum of smaller objects. For example, in Vk , any vector spaces
V and W of dimensions m and n decomposes as
V ⊗W =
mn
M
k.
i=1
Analogously, the fusion rules of F specify the decomposition of the fusion of two τ particles:
τ × τ = 1 + τ.
The simple objects of Vk clearly consist exclusively of the one dimensional vector space k, while the simple objects of F are
{1, τ}.
9
Braid Group Representations and non-triviality of UMTCs
The preceeding section should have conferred the sense that UMTCs are in a sense categories that behave like the category
of vector spaces, while at the same time UMTCs capture the basic data of anyonic statistics in a TQFT. While both of these
form examples of a UMTC, they can be seen to lie on opposite ends of a vast spectrum of complexity in a UMTC: the former
is in a certain sense trivial, while the latter is very interesting. To make this precise, consider the braiding maps in Vk
V ⊗W → W⊗V
given by the familiar map a ⊗ b → b ⊗ a, then this map squares to the identity. It follows that if in our TQFT we think of V
and W as anyonic excitations, each with their worldlines (See Figure 4), then V ⊗ W would model the fused particle i.e. the
composite subsystem consisting of the two particles. The braiding map then models the action on the fusion space when we
Figure 4: From [12]
twist one particle about the other; following this logic through, if we try to represent the braid group generator σV,W that
crosses the two strands corresponding to the worldlines of V and W (see appendix A), since this braid map squares to the
identity, we have in effect limited ourselves to a highly constrained representation of the braid group B2 . It follows that a
UMTC such as Vk does not model any interesting anyonic exchange statistics. Conversely, F has braid map Rτ,τ that yields
a highly non-trivial braid group representation (as referenced in Section 2, this representation is computationally universal).
The above discussion illustrates that in order to find UMTCs modeling non-trivial TQFTs, we have to go outside the
scope of classical algebra. One way to realize this principle is via the correspondence between certain TQFT and suitable
categories of representations of a Quantum Group. For a survey of ways to obtain UMTCs in this way, see [9].
Unitarity, Modularity and TQFTs
Together with Unitarity which guarantees a given hermitian structure on End(V) for any object V ∈ C , the Modularity
condition (which we have not treated here) guarantees that to each UMTC we can uniquely associate a TQFT; the precise
construction of a field theory from the abstract algebraic description of a tensor category is beyond the scope of this report.
We instead refer the interested reader to now-standard reference [12].
References
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[2] Rowell and Wang, Degeneracy Implies Non-abelian Statistics http://arxiv.org/pdf/1508.04793.pdf, 2015
[3] Bruillard, NG, Rowell, and Wang On classification of modular categories by rank http://arxiv.org/pdf/1507.05139.pdf,
2015
[4] Rowell, Stong and Wang, On classification of Modular Tensor Categories http://arxiv.org/pdf/0712.1377v4.pdf, 2009
[5] Naidu and Rowell, A finiteness property for braided fusion categories http://arxiv.org/pdf/0903.4157.pdf, 2009
[6] Wang, Topological Quantum Computation, 2008
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10
[8] Freedman, Larsen and Wang, A Modular Functor Which
http://stationq.cnsi.ucsb.edu/ freedman/Publications/76.pdf, 2001
is
Universal
for
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[13] V. Tuarev, Quantum Invariants of Knots and 3-Manifolds, 2nd. Edition, De Gruyter Studies in Mathematics 18, April 20
[14] https://www.encyclopediaofmath.org/index.php/Braid theory
[15] arXiv:0902.3275
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